1. Field of the Invention
The present invention relates to a piezoelectric thin-film resonator.
2. Description of the Related Art
Piezoelectric thin film resonators include a thin film unit disposed on a substrate. The thin film unit has a piezoelectric film sandwiched by upper and lower electrodes. An overlapping portion of the upper and lower electrodes functions as a vibration unit, which is acoustically isolated from the substrate. When an alternating voltage is applied to the vibration unit, the vibration unit operates in a thickness extensional vibration mode at a frequency for which the thickness of the vibration unit is approximately equal to a half wavelength. At the same time, Lamb waves are excited so as to propagate in a planar direction of the piezoelectric film. When the planar shape of the vibration unit is square, standing waves occur between the opposing sides of the vibration unit, and therefore, a spurious component is generated.
As an example of the related art that prevents the formation of a spurious component of the Lamb waves, Japanese Unexamined Patent Application Publication No. 2000-332568 describes a vibration unit having a non-square, irregular polygonal planar shape.
In addition, WO99/37023 describes technology in which an additional film is provided in a peripheral portion of a vibration unit.
Furthermore, PCT Japanese Translation Patent Publication No. 2003-505906 describes technology in which the thickness of the end portions of an electrode film is reduced so as to be thinner than the middle portion, thereby forming a stepped portion.
In the technology described in Japanese Unexamined Patent Application Publication No. 2000-332568, spurious components of the Lamb waves having a long wavelength and a short wavelength can be prevented. However, the Q value of resonance is disadvantageously decreased. In addition, if the vibration unit has a non-rectangular, irregular polygonal planar shape, it is difficult to reduce the size of the vibration unit.
In the technology described in WO99/37023, the resonant frequency of the peripheral portion of the vibration unit is decreased by using an additional film. In this way, frequency-increasing energy that relatively increases the resonance frequency of the vibration unit is trapped. Accordingly, the problem of the formation of a spurious component of the Lamb waves is not addressed.
Furthermore, in the technology described in PCT Japanese Translation Patent Publication No. 2003-505906, a spurious component of the Lamb waves having a long wavelength can be suppressed. However, a spurious component of the Lamb waves having a short wavelength cannot be suppressed.